Essential oils are important aroma substances in the foodstuff industry. Thus, for example, cold-pressed oils from citrus fruits are used to a large extent for the production of essences for the beverage industry, as well as for the aromatization of bakery products. These essential oils frequently contain terpene hydrocarbons of the mono- and sesquiterpene series which have only a limited storage stability and are thermolabile and, in addition, display a smaller aroma intensity than the actual aroma material which is preponderantly composed of volatile oxygen-containing compounds, such as aldehydes, ketones, esters, acids, phenols, alcohols and lactones. For these reasons, the removal of the terpenes is an important step for the improvement of the storage stability and for intensifying the aroma intensity of essential oils.
Furthermore, by the removal of the non-polar terpene hydrocarbons, the water-solubility of the essential oils is improved, which is of particular importance for the beverage industry.
A number of processes are known for the removal of terpenes from of essential oils which, for the separation of the terpenes, utilize differences in the vapor pressure, in the polarity or in the solubility of the terpene components in comparison with the oxygen-containing compounds. All these processes possess certain disadvantages which manifest themselves either in the quality of the product, in the process costs or in the yields. Thus, for example, there has been described the dissolving of essential oils in aqueous alcohols, the terpenes thereby separating out, followed by the recovery of the desired aroma fractions by salting out or by liquid-liquid extraction. In the case of this process, the separation action and the yields are not satisfactory. Furthermore, depending upon the nature of the extraction agent used, technical or environmental problems can arise.
Chromatography is a further known method, for example for the enrichment of citrus oils. These processes are very laborious and expensive since it is necessary to work with very dilute solutions. Furthermore, in the case of the subsequent evaporation of the solutions, there is a danger of the thermal decomposition of the sensitive component materials or of the loss of the low boiling aroma materials.
The removal of terpenes by means of rectification or distillation in a vacuum, as well as steam distillation, are also widely used methods. These processes do not provide high quality products since the aroma components are considerably damaged by the thermal stressing.
In contradistinction thereto, processes of high pressure extraction for the enrichment of essential oils, which recently became known, are substantially more gentle. Thus, for example, in Chem. Ing. Tech., 56, 794/1984, there is described a process for the removal of terpenes from citrus oils in which the citrus oils are subjected to a countercurrent extraction with carbon dioxide at 70 to 90 bar and at about 55.degree. to 85.degree. C. in a counterflow column to which is applied a temperature gradient. With the help of countercurrent extraction there can be achieved either high rates of enrichment or high yields but not both together (cf. Food Technology, 6, 145/1988) since either the selectivity of the process is small or the loading of the carbon dioxide with terpenes is low.
Finally, in U.S. Pat. No. 4,647,466, there is described a process for the extraction of readily volatile oxygen-containing materials, such as ethyl butyrate or hexanal, from citrus oils with the help of compressed gases, limonene thereby being enriched. However, since citrus oils, for example orange oils, consist of up to 95% limonene, very large amounts of carbon dioxide or long extraction times are necessary for carrying out the process in order to remove from the aroma oil a high proportion of limonene with the necessary selectivity.